Pipelines with an internal pipe are used, for example, if a hot-water system in which the water can circulate inside the feed line is to be installed in a home or business premises, in order to have hot water available as quickly as possible after turning on the tap. Such a system is disclosed, for example, in EP 0 674 137 A1. This publication describes a hot-water supply installation with a hot-water ring main which runs through a water heater and in which the water circulates in circulation lines of substantially smaller cross section.
The circulation line may be made of flexible material, i.e. in the most simple case may be a plastic hose which has been pushed into the water pipe, or may bear fixedly against or be attached to the inner wall of the outer pipeline.
A similar system is disclosed in DE 35 42 374 A1. In this system, T-pieces, by means of which not only the outer pipes but also the inner pipes can be connected to one another, are used as the pipe connectors. The connection takes place in each case via muffs, by which the pipelines, which are attached to the pipe connector in the direction of flow of the fluid, are inserted via the muffs. The circulation lines are connected in the fitting by adhesive bonding or shrink fitting. For this purpose, a T-piece is also provided in the interior, which T-piece is branched in the same way as the outer profile of the pipe and is attached, via a web, in the branch connection piece. Absolutely rigid fixing of the inner T-piece is impossible, since further webs would considerably restrict the pipe cross section in the fitting. Connecting the circulation line to the inner T-piece is therefore extremely complicated, in particular because the inner T-piece does not assume a defined position inside the fitting. It is therefore also consciously accepted that the connection location will not be absolutely leaktight in the interior. Although water losses at the connection location play only a subordinate role, since the water is returned, nevertheless heated water is "lost", so that the efficiency of the system is reduced. Moreover, owing to the complicated form of attachment, assembly time is extremely high.